Stretchable, biocompatible rubbers may yield novel implantable or wearable energy harvesting systems. As one example, decreasing power requirements for mobile electronic devices open the possibility of charging such devices by continuously extracting otherwise wasted energy from the environment. Such charging could be done with or without additional energy supplied by a battery.
Another attractive possibility is that of utilizing work produced by the human body via everyday activities, such as breathing or walking. The heel strike during walking is a particularly rich source of energy, with 67 watts of power available from a brisk walker. Harvesting even 1-5% of that power would be sufficient to run many body-worn devices such as mobile phones. Similarly, it has been shown that lung motion by breathing can generate up to 1 W of power. If this power were harvested into charging a pacemaker battery, for example, it may increase the time required between battery replacement surgeries for patients.
Crystalline piezoelectric materials are promising materials for electromechanical energy conversion technologies. These materials become electrically polarized when subjected to a mechanical stress, and conversely experience a strain in response to an applied electric field, the strain being in proportion to the strength of that field. Single-crystal perovskites, such as lead zirconate titanate (PZT), are an exceptionally efficient class of energy conversion materials. Indeed, conversion of mechanical to electrical energy with efficiencies above 80% has been demonstrated using PZT piezoelectric cantilevers operated near resonance.
Epitaxial growth of such crystalline materials depends on the use of rigid, inorganic host substrates, as well as high temperature deposition processes. For example, rf sputtering at 600° C. has been shown to yield single-crystal films of PZT over large areas with excellent compositional control when deposited on MgO or SrTiO3 substrates. However, next-generation applications, such as wearable energy harvesting systems, may require the piezoelectric materials to be flexible, lightweight, and biocompatible. The flexible piezoelectric polymer polyvinylidene difluoride (PVDF) has been used for applications such as shoe-sole power generators and implantable breath harvesting.